Second harmonic generation in air-exposed few-layer black phosphorus

Black phosphorus (BP) shows significant saturable absorption and other unique properties in nonlinear optical effects, and it can be used for nonlinear optical elements and other photonic devices. However, the second harmonic generation (SHG) emission is prohibited due to its inversion symmetry. Here we report emerging SHG emission in BP by exposing BP to oxygen so as to break the inversion symmetry. The intensity of SHG signal shows a dependence on thickness and exposure time. It is interesting to find that the thinner BP flakes have faster SHG intensity change rate. Our results demonstrate that the intensity of SHG signal is attributed to oxidation degree. Air exposure also causes a blue shift of photoluminescence (PL) peaks, which is consistent with our calculation results. Our study demonstrates a feasible approach to modulate the SHG emission of BP, which may provide insights to other inversion symmetric crystals like BP.

Automated summary

Black phosphorus (BP) exhibits significant saturable absorption and other unique properties in nonlinear optical effects, making it suitable for nonlinear optical elements and photonic devices. However, its inversion symmetry prohibits second harmonic generation (SHG) emission. In this study, we introduce emerging SHG emission in BP by exposing it to oxygen to break the inversion symmetry. The intensity of SHG signal is dependent on the thickness and exposure time, with thinner BP flakes showing a faster SHG intensity change rate. This study provides insights into modulating the SHG emission of BP and potentially other inversion symmetric crystals.